1. Field of the Invention
The present invention relates to a wavelength converter for use in wavelength division multiplex (WDM) optical communication, and more particularly for use in high-speed WDM optical communication, and to a method for removing wavelength chirping from such a wavelength converter.
2. Related Arts
In recent years, WDM optical communication has been developed into a mass optical communication system. With this WDM optical communication system, each wavelength in a light signal is allocated to an individual communication channel, and therefore, in order for a signal to be transmitted or to be received from another channel, it is necessary for the wavelength of the original signal string to be converted into a wavelength that is suitable for a second channel. For conversion of a signal wavelength using a conventional optical device, a light signal must first be converted into an electrical signal, which then must be converted into a light signal having a different wavelength.
On the other hand, research has recently been performed employing a device that is so designed that it converts optical signal wavelengths directly, without converting them to electrical signals first. And there has been some discussion of proposed interactive devices, such as a symmetrical Mach Zender device and a deflected light isolation device. There has also been a report of the use as an optical switch of the symmetrical Mach Zender type wavelength converter ("Applied Physics Letters," by S. Nakamura, et. al., vol. 67, pp. 2445) that has one of the same features as has a wavelength converter: it produces output signal pulses having wavelengths that differ from those of input signal pulses. This symmetrical Mach Zender type wavelength converter has the configuration shown in FIG. 1, in which two waveguides that can produce changes in nonlinear refractive indices are arranged along the respective arms of a Mach Zender interferometer. If a signal having a pulse width that is sufficiently shorter than .DELTA.t is input at time t.sub.1 and time t.sub.1 +.DELTA.t, a light pulse is output that has a first transition time t.sub.1 and a last transition time t.sub.1 +.DELTA.t. Further, a wavelength converter having the above described structure, with the exception that an optical delay circuit is arranged differently, is disclosed by S. Nakamura, the present inventor, in Japanese Unexamined Patent Publication No. Hei 7-199240.
The operating principle of an XPM type wavelength converter differs from that of a symmetrical Mach Zender wavelength converter in that it produces an output signal having substantially the same pulse as that of an input signal ("Journal of Lightwave Technology," by T. Durhuus, et. al., vol. 14, pp. 942). However, the XPM wavelength converter, as well as the symmetrical Mach Zender wavelength converter, has a configuration in which two waveguides that produce changes in nonlinear refractive indices are arranged along the respective arms of a Mach Zender interferometer.
One problem arising from the use of these wavelength converters is that an output signal pulse produces a great deal of wavelength chirping. The wavelength chirping occurs because the changes in the nonlinear refractive indices at the waveguides alter the phase of a signal pulse light. A specific example of the wavelength chirping produced by the XPM wavelength converter is reported by Durhuus et al. in the above reference.